1. Field of the Invention
The present invention relates to a radiation-curing aqueous ink composition used preferably for inkjet recording and to an inkjet recording method. Specifically, the invention relates to a radiation-curing aqueous ink composition which is excellent in jetting reliability and curable with high sensitivity by irradiation with a radiation, a radiation-curing aqueous ink composition for inkjet recording, and an inkjet recording method.
2. Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photoreceptor drum by electrically charging and exposing to light is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.
With regard to the inkjet system, on the other hand, the equipment is inexpensive, and since an image is formed directly on a recording medium by jetting an ink only on a required image area, there is an advantage that the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
An ink composition that can be cured by irradiation with radiations such as ultraviolet rays (radiation-curing ink composition), for example an ink composition for inkjet recording, is required to be curable with high sensitivity to form a high-quality image.
Since high curability by irradiation with radiations is conferred by achieving higher sensitivity, an image recording method is obtained which has a large number of benefits such as a reduction in power consumption, longer lifetime due to a decrease in the load on a radiation generator, and achievement of sufficient curing which enables prevention of reduction in volatilization of uncured low molecular weight material and prevention of reduction in the strength of a formed image.
In particular, the radiation-curing aqueous ink can be preferably used in image printing, in pretreatment for conferring printability on a recording medium and in posttreatment for protection/modification of printed images, is excellent in safety due to use of water as a main solvent, is able to form high-quality images because of a small content of a curable component, is applicable to high-density inkjet recording due to low viscosity, and is thus an art having many excellent features and possibilities.
Fundamental constituent materials of the radiation-curing aqueous ink are water, a polymerizable material, a polymerization initiator that initiates polymerization by generating a radical or the like by a radiation, and a colorant (a pigment or a dye). Among them, the polymerizable material and the polymerization initiator are prepared either in an emulsified state or in a solution state with water solubility conferred by suitable substituents. Examples of the radiation-curing ink wherein the polymerizable material and the polymerization initiator are water-soluble include those described in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-307199, and examples of the active radiation-curing ink wherein the polymerizable material and the polymerization initiator are present in an emulsified state include those described in, for example, JP-A No. 2004-285216, and ink compositions for inkjet recording which produce films excellent in strength, flexibility and adhesiveness by light irradiation are described therein.
As photopolymerization initiators contained in UV-curing ink compositions for inkjet recording, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethylanthraquinone are generally used (see Bruce M. Monroe et al., Chemical Review, Vol. 93, pp. 435-448 (1993)). However, photopolymerizable compositions containing these photopolymerization initiators have low sensitivity and thus require a long time for image-wise exposure in forming images. Therefore, if there is slight vibration during operation, images with good image quality cannot be obtained in the case where fine images are desired to be formed. Further, since a long exposure time is accompanied by an increase in energy radiation in the exposure step, a measure against radiation of the great heat resulting therefrom is needed.
The ink composition used in the inkjet recording system needs various properties so that during storage, physical properties are not changed or precipitates are not generated (solution stability) and nozzle clogging is not generated (charge stability).
As a method of improving the radiation sensitivity of the radiation-curing polymerizable composition, use of various polymerization initiation systems is disclosed (Bruce M. Monroe et al., Chemical Review, Vol. 93, pp. 435-448 (1993), U.S. Pat. No. 4,134,813, JP-A No. 1-253731, and JP-A 6-308727). JP-A No. 2005-307199 makes use of initiators containing water-soluble acylphosphine oxides and thioxanthones applicable to radiation-curing aqueous ink compositions. However, there is none of examples where an initiator satisfying sufficient sensitivity to scanning exposure, storage stability, and charge stability is used in the aqueous ink composition for inkjet recording.
Accordingly, there is an earnest desire for a radiation-curing aqueous ink composition usable preferably in inkjet recording, which can, by irradiation even with a low-power radiation, be cured highly sensitively to form high-quality images and is excellent in storage stability and jetting stability.